Dienes, especially isoprene, are useful as monomers for the manufacture of synthetic rubbers. Isoprene is primarily used to make cis-polyisoprene which is a stereospecific rubber having the same segmeric unit as natural rubber. Several fundamental processes have been used to construct the isoprene C.sub.5 skeleton from smaller carbon units. These processes are not commercially accepted in that there are numerous problems associated with each particular synthesis route. One route involves condensing acetylene and acetone followed by hydrogenation and dehydration. Another route involves as a first step the reaction between formaldehyde and isobutylene, and in a subsequent step the intermediate derivative is catalytically cracked at elevated temperatures. See for example, French Pat. No. 1,294,716; Chem. Abstracts 57:15309.
European Patent Application No. 80449 based on U.S. application Ser. No. 315,803 discloses the synthesis of isoprene from linear butenes wherein mixed linear butenes are catalytically isomerized to a mixture of cis- and trans-butene-2, and then hydroformylating the butene-2 mixture to 2-methylbutanal (2MBA) in the presence of a homogeneous rhodium catalyst and organic ligand. The 2MBA is then dehydrated to isoprene in the presence of acidic heterogeneous catalysts at elevated temperatures. This European patent application discloses a preferred catalyst for the dehydration step as a boron phosphate which is described in British Patent No. 1,385,348. The dehydration reaction is endothermic, and under preferred conditions, the reaction is performed in the vapor phase over a fixed bed of catalyst at elevated temperatures from about 200.degree. to about 400.degree. C. This patent application, however, does not disclose the length of time the catalyst performs at such selectivities and conversions (lifetime). Commercial production of isoprene via the aldehyde dehydration route has not been established since the dehydration catalyst is known to have short lifetimes which limit its utility in commercial applications.
U.K. Patent No. 1,385,348 relates to the conversion of aldehydes to dienes with conjugated double bonds. This British patent recites that particularly preferred acid dehydration catalysts are mixed acid anhydrides, for example, boron phosphate, silicoborate or silicotitanate. In these mixed acid anhydrides the two acid components may be present in a stoichiometric ratio or, alternatively, one of the two components may be present in excess. Boron phosphate is particularly preferred. Further, U.K. No. 1,385,348 states that it is advantageous for the boron phosphate to contain an excess of from 1 to 10% by weight of phosphoric acid. The examples provided in the British patent utilizes a boron phosphate containing an excess of phosphoric acid. The examples recite results with conversions as high as 92.9% and selectivities as high as 68.4%. However, there is no discussion or information relating to the duration of such conversions and selectivities and/or the number of regenerations required during any particular time period.
U.K. Patent No. 2,093,060 relates to the preparation of substituted dienes, especially isoprene, from a corresponding carbonyl compound in which magnesium ammonium phosphate or its decomposition products are used as the dehydration catalyst. However, there is no data relating to the duration of catalyst activity.
A disadvantage associated with known catalysts to dehydrate aldehydes is that catalyst life depends on many factors which include catalyst composition and structure, catalyst activity, operating temperatures and coke deposition. Coke deposition is understood to denote coke (carbonaceous) deposits formed on the catalyst during the dehydration reaction. As stated earlier, no commercial process based on said technique has been developed so far, since there is no catalyst with selectivity and stability to justify a commercial process.
The use of boron phosphate as a catalyst for the dehydration of alcohols such as 2-butanol and 2-methyl-2-butanol is known. See Jewur and Moffat, Journal of Catalysis, 57, 167-176 (1979). The problems associated with an aldehyde dehydration are different and more difficult to overcome than those found in alcohol dehydrations. For example, the boron phosphate dehydration of 2-methyl-2-butanol yields only 2-methyl-2-butene and 2-methyl-1-butene, while dehydration of 2MBA yields primarily methylisopropylketone, 2-methyl-2-butene, 2-methyl-1-butene and isoprene. It is the production of the conjugated diolefin, isoprene, that makes the aldehyde dehydrations so difficult, since this highly reactive monomer is known to form dimers and/or polymerize in the presence of acid catalysts.
In addition, aldehydes such as 2MBA are known to undergo aldol condensation. This is a reaction between two molecules of an aliphatic aldehyde whereby a 3-hydroxyaldehyde is formed. Dehydration of the 3-hydroxyaldehyde results in the formation of terpenes, a highly undesirable by-product that can coke and deactivate the catalyst. Due to these and other differences, catalysts suitable for long term dehydration of alcohols have not been found acceptable for aldehyde dehydration.
One aspect of this invention is directed to the use of graphite which is in intimate physical admixture with the boron phosphate. Graphite has numerous known uses including utility as a carrier for catalysts and as a lubricant during catalyst pellet formation. See U.S. Pat. No. 1,841,055. The prior art does not suggest, disclose, or appreciate that the presence of graphite in a boron phosphate catalyst will unexpectedly enhance the viable lifetime of the catalyst in a 2MBA to isoprene dehydration. In addition to the P/B ratios and graphite, the instant invention is also directed to the ammonia or amine modification wherein the ammonia or amine is added during the boron phosphate preparation or subsequent thereto. The ammonia or amine is added in the range of 0.1 to 10 mole percent per mole of boron phosphate.
A portion of the instant invention is directed to a catalyst of high selectivity and low coke deposition in conjunction with extended catalyst lifetimes. The prior art does not suggest or disclose a specific catalyst or process for the dehydration of aldehydes to dienes which would be suitable for commercial application. The catalyst of this invention is boron phosphate wherein the molar ratio of P/B is initially less than 1.0 with from 0.1 to 10 weight percent graphite in physical admixture with or without 0.1 to 10 mole percent ammonia or amine.